If you apply too much caulk, it doesn't look good. If you don't apply enough caulk, you aren't sealing. The painters tape makes sure the excess caulk gets removed.

Tape just a hair below the top of the baseboard. You should be able to fit a coin standing up, but that's about it. You can also put a bit of tape on the wall just above the baseboard. Your caulk tube should have a relatively small hole to better control the flow of caulk.